A submarine branching unit (BU) is an underwater device of a submarine optical transmission system, and a main function of the submarine branching unit is to implement fiber pair interconnection of three ports of the BU by means of a connection configuration of fibers inside the BU. The three ports of the BU are separately connected to a submarine cable and an underwater repeater, so that service transmission among multiple stations of the submarine optical transmission system is implemented. The BU is a key device for forming the complex submarine optical transmission system.
When service transmission capacities of a trunk and a branch of the submarine optical transmission system are relatively small, because of a confidentiality requirement for service transmission, it is required that service transmission between trunks cannot be wrap connected to a branch landing station. Usually, an optical add/drop multiplexer (OADM) optical module needs to be integrated inside the BU, which is referred to as an OADM BU, and service transmission between a trunk and a branch is carried by adding and dropping a wavelength (or waveband).
For a submarine cable system integrating with an OADM BU, when a cable cut fault occurs in a trunk or a branch, an actual quantity of channels for transmission in a fiber is reduced, but output power of a submarine repeater substantially remains unchanged, which leads to an increase in single-wavelength optical power. As a result, a non-linear effect of transmission in the fiber is significantly increased, and normal service transmission cannot be sustained in a submarine cable section on a non-fault side; normal service transmission cannot be sustained even in a fault rectification period. To resolve this problem, people propose various solutions, which are also referred to as an OADM BU redundancy technology. A redundancy technology has become a basic requirement for the submarine cable optical transmission system integrating with the OADM BU.
A redundancy solution provided by an OADM BU device helps reduce an economic loss of an operator that results from a cable cut fault. In a currently used OADM BU redundancy technology, a repeater is generally integrated inside an OADM BU, and a redundancy function is implemented by using power of amplified spontaneous emission (ASE) noise generated by the repeater. Integrating with a repeater usually refers to integrating with a repeater in both a pass-through direction and an add direction inside the OADM BU.
In the solution of integrating a repeater inside the OADM BU, when a cable cut fault occurs in a trunk of the submarine cable system, the OADM BU can still obtain power supply from a submarine cable power supply line, and service optical power that is lost because of trunk cable cut is compensated for by using ASE noise generated by an erbium-doped fiber amplifier (EDFA) integrated on a pass-through optical path inside the OADM BU. The ASE noise passes through an optical filter (Wavelength Blocking Filter, WBF), is combined with an add optical path service by using an optical coupler, and is transmitted in a submarine cable line, so as to ensure that a normal optical transmission service is sustained between a branch and a trunk in neither of which a fault occurs, thereby implementing branch redundancy. When a cable cut fault occurs in a branch of the submarine cable system, service optical power that is lost because of branch cable cut is compensated for by using ASE noise generated by an EDFA integrated in an Add optical path inside the OADM BU. The ASE noise passes through a band pass filter (BPF) and is combined with service light of a pass-through optical path, so as to ensure that a normal optical transmission service is sustained in a trunk in which no fault occurs, thereby implementing trunk redundancy.
The redundancy solution of the OADM BU integrating with a repeater has the following disadvantages: 1. Although the ASE noise generated by the repeater can be used to compensate for service signal optical power that is lost when a fault occurs, an optical signal-to-noise ratio of a service optical signal may be reduced when noise optical power is transmitted together with signal light, especially for a long-distance optical transmission system, cost of the optical signal-to-noise ratio (OSNR) is excessively high, and a redundancy effect is limited; 2. Compared with a common OADM BU product, an OADM BU system integrating with the repeater is high in costs and complex in optical path, and overall product reliability is reduced; 3. Structure space of the OADM BU integrating with the repeater increases, and a structural design is complex.